Carbohydrate supplements to the callus culture medium modify the growth of potato tuber moth larvae feeding on Lycopersicon chmielewskii callus

Lycopersicon esculentum and L. chmielewskii are respectively susceptible and resistant to the potato tuber moth (Phthorimaea operculella Zeller) in the field. Feeding bioassays were conducted with the herbivore caterpillars reared on callus derived from both tomato species and grown in vitro, and the influence of carbohydrate supplements to the callus culture medium, on the insect's feeding behavior was investigated. Newly-hatched larvae fed with L. esculentum or L. chmielewskii callus raised on a medium with 88 mM sucrose, reached a weight of 12-15 mg and 1.5-3.0 mg, respectively, within 9 days. Restriction of larval weight increase in insects reared on L. chmielewskii callus, disappeared when the host tissue was transferred 24 h prior to the callus-insect assay to a medium supplemented with 264 mM of either sucrose, glucose, fructose or mannose. The capability of L. chmielewskii callus to restrict growth of larvae was restored in host tissue retransferred from a medium with 264 mM sucrose to a 24-h incubation on one supplemented with 264 mM of either mannitol, sorbitol, glycerol or myo-inositol, before the callus-insect bioassay. The larval growth response remained unaltered by callus incubated on a medium with 264 mM xylose. The ameliorating effect on insect growth of high sucrose in the callus medium was not due to sucrose as an ingredient of the insect's diet. The diverse response of L. chmielewskii callus, and its dependence on the type of carbohydrate in the medium, rule out effects of these substances as nonspecific medium osmotica. The swift callus responses to carbohydrates (within hours of a change in medium composition), as reflected in the insect's growth, were not accompanied by visible morphological variations in the host tissue. We suggest that suppression by high levels of exogenously applied saccharides and derepression by exogenous polyols and myo-inositol of the impedement to growth of the potato tuber moth larva, reflect the existence in L. chmielewskii of a carbon metabolic control mechanism of gene expression whose products affect insect growth.